In hydrometallurgical applications gas is usually fed to a stirred tank reactor below the agitator through a plain pipe. Gas is then dispersed to fine bubbles with powerful mixing. Required mixing power is typically at the range of 0.5-2 kW/m3. Another option is to use some kind of a sparging means at the end of gas feed pipe. These spargers can be just drilled holes in a ring shape pipe or be made of some porous material. Idea is that feed gas is broken into smaller bubbles before they hit the agitator (impeller which creates the main flow pattern in the tank). This reduces slightly the power required for mixing. At hydrometallurgical applications this kind of devices are not very suitable, since they tend to block up easily. Further, the problem is that a substantially high pressure for the gas feed is needed because of a great pressure loss caused by the small holes or porous material.
A stirred tank reactor for gas-liquid mass transfer is known e.g. from document U.S. Pat. No. 5,108,662 which discloses a stirred tank reactor. A motor-driven drive shaft extends vertically in a reactor tank. A downward pumping axial flow impeller is attached to the drive shaft to create a main flow pattern in the reactor tank. A gas inlet is arranged to supply gas into the tank below the axial flow impeller to be dispersed to the liquid. The document proposes a separate mixing system for gas dispersion. The system can be placed outside the reactor tank or inside the tank at the surface. The problem is that the construction of this known gas sparging mechanism is complex and requires installation of at least two mixer mechanisms and electric motors for one stirred tank reactor. Further, only part of the fed gas enters the gas sparger mechanism.